BitField: Add an explicit Assign method.

This is useful when doing crazy stuff like inheriting from BitField.

src/common/bit_field.h

@@ -142,7 +142,7 @@ public:
 
     __forceinline BitField& operator=(T val)
     {
-        storage = (storage & ~GetMask()) | (((StorageType)val << position) & GetMask());
+        Assign(val);
         return *this;
     }
 
@@ -151,6 +151,10 @@ public:
         return Value();
     }
 
+    __forceinline void Assign(const T& value) {
+        storage = (storage & ~GetMask()) | (((StorageType)value << position) & GetMask());
+    }
+
     __forceinline T Value() const
     {
         if (std::numeric_limits<T>::is_signed)